We will study the postural aspects of the visual-vestibular interaction and we will apply our findings to the postural problems of the aged. As a first specific aim, we will determine if there is a non- Gaussian component to postural sway, which is a normal instability of stance. We will measure this sway with a platform which has pressure transducers under each corner. We will test the skewness and kurtosis of postural sway for statistical significance. On this information, we will base all subsequent statistical analysis. As a second specific aim, we will test the hypothesis that postural sway is described by a second order, linear, autonomous, stochastic differential equation. We do this under three conditions: 1) Stationary stimuli, 2) transient stimuli, and 3) sinussoidal stimuli. To produce the last two conditions, we will move either the platform on which the subject is standing or the visual surround. Unlike current theories, this model provides a link between postural sway and postural perturbations. In addition, it also provides a statistically more efficient and more complete mechanisms for parameterizing the postural system than current procedures. As a third specific aim, we will test a nonlinear addition to the model by analyzing effects which depend on the amplitude of the perturbations. This nonlinear component adds instabilities to the model which can be interpreted as falls. This would then be the basis for further study on the relationship between postural sway measurements and the the susceptability to falls. As a fourth specific aim, we will demonstrate the postural sway in abnormal in the aged and that this abnormal sway is consistent with their higher incidence of falls. To do this, we will use the parametric procedures developed in the earlier part of this work and compare the results of a geriatric group to the findings of a control group which has been matched for height and weight. This will serve as the basis for a long term study of the relationship between postural sway measurements and falls in the aged.